1. Field of the Invention
This invention relates to the fields of networks with distributed intelligence, for providing, control and communication through electrical power lines in houses and businesses, and more particularly, to electrical coupling of communication in such network. This field is sometimes referred to as power line communications (PLC).
2. Prior Art
There are commercially available products and proposed systems which permit intelligent networking of a control device to other devices through the power lines in a house, business, manufacturing facility, or the like (collectively referred to herein as households). Often these devices are coupled into standard wall sockets, light switches, or the like and send signals through the power wiring of such households to a receiving device coupled into a socket or otherwise connected to the household wiring. Such signals would be, for example, a command to turn on or off a lamp, television or other appliance.
These devices operate, in general, on the principle of sending a coded message or signal to a receiving device at a high frequencies relative to the frequency of alternating current. (Standard alternating current operates at 60 Hz in North America, and 50 Hz in many other parts of the world.) In comparison, the PLC transmit signals in a substantially higher range, as is known in the art. Such devices utilize signals greater than 1 KHz and typically in the 5 to 150 KHz range at the present time. Accordingly, in theory (and in theory only) there is little or no interference between the electrical power system and the communications system. One of the systems for providing communications and control is described in U.S. Pat. No. 4,918,690. Another system is sold by BSR under the trademark X-10.
Small domestic consumers of electricity are usually supplied with single phase power which comprises two power lines 180.degree. out of phase with respect to each other, and a neutral (white coded wire). There is also a ground line (green coded wire or a bare wire) which is connected to the neutral wire at the power service entrance. Each of the two power lines carries approximately 110 to 120 volts with respect to the neutral line, and since the lines run 180.degree. out of phase, the total voltage across the two power lines is 220 volts (assuming 110 volts and not 120 volts is present on each of the power lines with respect to neutral). In wiring standard households and small businesses an attempt is usually made to place one half the 110 volt load on each of the power lines. This is simply done by running one of the two power lines and the neutral line to approximately one half of the outlets and light fixtures, and the other power line and the neutral line to the other half of the outlets and light fixtures. As a result, any one 110 volt outlet, light switch or light fixture is only in communications with one of the two power lines, and the neutral line. (There is a link between the two 110 volts lines at the power transformer feeding the household, but this is not adequate for typical PLC.) Most houses and businesses also have 220 volt outlets for certain appliances such as dryers and ranges. The two power lines along with the ground and/or neutral line are coupled to the outlet.
As a result of the two 110 volt power lines, a transmitter connected into a single 110 volt socket can only readily transmit its signals to other sockets and devices operating off the same power line. Thus, approximately one half of a household's sockets and appliances plugged therein or otherwise connected to the household wiring can only be controlled by a transmitter plugged into a wall socket.
To get around this problem, purchasers of these systems have had to provide coupling between the power lines at the service (meter) box or circuit breaker panel. For example, a capacitor is coupled between the power lines at the main power panel. These capacitors have a very low impedance at high frequencies and a very high impedance at low frequencies thus permitting only the high frequency control signals to cross between the two power lines. The installation of such a capacitor is not a simple task for a typical householder. Most people, rightly so, are fearful of touching the interior of these power boxes. Thus, a professional installer is often required to install the capacitor.
This and other drawbacks of the prior art are overcome by the present invention which is described below.